This research is a continuation of our studies of a novel method for measuring the rate of breakdown of muscle proteins in the intact organism, based on the determination of urinary 3-methylhistidine excretion. In this context we plan to explore the roles of protein synthesis and breakdown in the regulation of muscle protein content under various physiological and pathological states, including nutritional and hormonal treatments and conditions which lead to muscle wasting such as injury and infection. In vivo rates of muscle protein synthesis will be determined by use of constant intravenous 14C-amino acid labelling; rates of protein synthesis and breakdown will be determined also in excised muscle. These studies will consolidate the rates of synthesis and breakdown in the regulation of muscle protein and further establish the value of urinary 3-methylhistidine measurements as a direct index of muscle protein breakdown in states of health and disease.